Preparation and HPLC applications of rigid macroporous organic polymer monoliths
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J. Sep. Sci. 2004, 27, 747766 www.jss-journal.de i 2004WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim
Department of Chemistry,University of California, Berkeley,CA 94720-1460, USA
Preparation and HPLC applications of rigidmacroporous organic polymermonoliths
Rigid porous polymer monoliths are a new class of materials that emerged in theearly 1990s. These monolithic materials are typically prepared using a simple moldingprocess carried out within the confines of a closed mold. For example, polymerizationof a mixture comprising monomers, free-radical initiator, and porogenic solventaffords macroporous materials with large through-pores that enable applications in arapid flow-through mode. The versatility of the preparation technique is demonstratedby its use with hydrophobic, hydrophilic, ionizable, and zwitterionic monomers. Sever-al system variables can be used to control the porous properties of the monolith overa broad range and to mediate the hydrodynamic properties of the monolithic devices.A variety of methods such as direct copolymerization of functional monomers, chemi-cal modification of reactive groups, and grafting of pore surface with selected polymerchains is available for the control of surface chemistry. Since all the mobile phasemust flow through the monolith, the convection considerably accelerates mass trans-port within the molded material, and the monolithic devices perform well, even at veryhigh flow rates. The applications of polymeric monolithic materials are demonstratedmostly on the separations in the HPLC mode, although CEC, gas chromatography,enzyme immobilization, molecular recognition, advanced detection systems, andmicrofluidic devices are also mentioned.
Key Words: Polymeric monoliths; Preparation; Modification; Application; Stationary phase; Sep-aration; HPLC; CEC;
Received: January 20, 2004; revised: April 20, 2004; accepted: April 20, 2004
Monoliths are separation media in the format that can becompared to a single large particle that does not containinterparticular voids typical of packed beds. The firstattempts to make single-piece separation media dateback to the late 1960s and early 1970s. For example,highly swollen monolithic polymer gel was prepared by
free-radical polymerization of an aqueous solution of 2-hydroxyethyl methacrylate with 0.2% ethylene dimeth-acrylate (crosslinking monomer), inserted into a glasstube, and used for size-exclusion chromatography in 1967. Unfortunately, the effectiveness of fractionation wasrather low. Another early approach involved open-porepolyurethane foams prepared in situ . In contrast tothe hydrogel, the permeability of these monoliths wasexcellent. However, excessive swelling in some solventsand softness were deleterious characteristics that pre-vented their successful use in both liquid and gas chroma-tography. Macroporous discs  and compressed softpolyacrylamide gels  placed in a cartridge or columnrepresent other examples of monolithic materials. Theseelegant approaches have been described in detail in aseries of excellent review articles . The early1990s saw the development of another category of rigidmacroporousmonoliths formed by a very simple moldingprocess in which a mixture of monomers and solvent waspolymerized and immediately used within a closed tube orother container under carefully controlled conditions .Since porous inorganic materials are very popular sup-ports widely used in catalysis and chromatography ,monoliths prepared from silica were developed almostsimultaneously with the organic polymers [19, 20].
Correspondence: Frantisek Svec, Department of Chemistry,University of California, Berkeley, CA 94720-1460, USA.Phone: +1 510 643 3168. Fax: +1 510 643 3079.E-mail: email@example.com.
Abbreviations: HPLC, high performance liquid chromatography;CEC, capillary electrochromatography; PEEK, poly(ether-ether-ketone); AMPS, 2-acrylamido-2-methyl-1-propanesulfonic acid;GMA, glycidyl methacrylate; VAL, 2-vinyl-4,4-dimethylazlactone;ST styrene; BuMA, butyl methacrylate; NIPAAm, N-isopropyl-acrylamide; AIBN, 2,29-azobisisobutyronitrile; TEMPO, 2,2,6,6-tetramethyl-1-pyperidinyloxy; carboxy-TEMPO, 4-carboxy-2,2,6,6-tetramethyl-1-piperidinyloxy; carboxy-PROXYL, 3-car-boxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy; TEMED, N,N,N,N-tet-ramethylethylenediamine; DEAE, diethylaminoethyl; THF, tetra-hydrofuran; HIC, hydrophobic interaction chromatography;LCST, lower critical solution temperature; ESI MS, electrosprayionization mass spectrometry; IP-RP-HPLC, ion-pair reversed-phase high-performance liquid chromatography; ODS, octade-cylsilica; SEC, size-exclusion chromatography.
Detailed accounts of these materials have been publishedrecently [16, 2123] and the newest developments arepresented elsewhere in this issue.
2 Macroporous polymersMacroporous polymers emerged in the late 1950s as aresult of the search for polymeric matrices suitable for themanufacture of ion-exchange resins with better osmoticshock resistance and faster kinetics. The history of theseinventions has been reviewed a short time ago . Incontrast to the polymers that require solvent swelling tobecome porous, macroporous polymers are characterizedby a permanent porous structure formed during their prep-aration that persists even in the dry state. Their internalstructure consists of numerous interconnected cavities(pores) of different sizes, and their structural rigidity issecured through extensive crosslinking. These polymersare typically produced as spherical beads by a suspensionpolymerization process . To achieve the desiredporosity, the polymerization mixture should contain both acrosslinking monomer and an inert diluent, the poro-gen . Solvating or non-solvating solvents for thepolymer that is formed, supercritical carbon dioxide, orsoluble non-crosslinked polymers or mixtures of suchpolymers and solvents have proven to be efficient poro-gens.
Macroporous polymers are finding numerous applicationsas both commodity and specialty materials. The formercategory includes ion-exchangers and adsorbents, sup-ports for solid phase synthesis, polymeric reagents, andcatalysts, while chromatographic packings fit well into thelatter . Although the vast majority of current macropor-ous beads are based on styrene-divinylbenzene copoly-mers, other monomers including acrylates, methacry-lates, vinylpyridines, vinylpyrrolidone, and vinyl acetatehave also been utilized .
While the suspension polymerization that affords macro-porous polymers has already been analyzed in the litera-ture many times , little was known until recently onhow to prepare macroporous polymers by bulk polymeri-zation within amold [17, 33, 34].
2.1 Preparation of rigid polymermonoliths
The preparation of rigid macroporous organic polymersproduced by a facile molding process is simple andstraightforward. The mold, typically a tube, is sealed atone end, filled with a polymerization mixture, and thensealed at the other end. The polymerization is then trig-gered, often by heating in a bath at a temperature of 55808C [17, 35, 36]. In addition to thermally initiated poly-merization, redox initiation has also been used .Another option, UV light initiation, can only be carried outin UV transparent molds such as glass tubes, fused silica
capillaries, and microfluidic chips . The seals arethen removed, the tube is provided with fittings, attachedto a pump, and a solvent is pumped through the monolithto remove the porogens and any other soluble compoundsthat remained in the pores after the polymerization wascompleted. A broad variety of tube sizes and materials,such as stainless steel, poly(ether-ether-ketone) (PEEK),glass, plastic microchips, and fused silica capillaries havebeen used as molds for the preparation of monoliths [37,38, 4050].
While the preparation of cylindrical monoliths with a homo-geneous porous structure in capillaries and tubes up to adiameter of about 1025 mm is readily achieved in a sin-gle polymerization step, larger size monoliths are some-what more difficult to prepare. Dissipation of the heat ofpolymerization is frequently slow and the exotherm maybe sufficient to increase substantially the reaction tem-perature, significantly accelerate the polymerization, andcause a rapid decomposition of the initiator. If this processis not controlled, monoliths with unpredictable radial andaxial gradients of porosity are obtained . However, theslow and gradual addition of the polymerization mixture tothe reaction vessel in which the polymerization reactionproceeds continuously minimizes the heat production andallows the preparation of very large diameter monolithswith homogeneous porous structures. Another elegantmethod that helps to solve the problem of heat dissipationhas been demonstrated recently . Using analysis ofthe heat release during the polymerization, Podgornik atal. derived a mathematical model for the prediction of themaximum thickness of the monolith that can be preparedin a single step without affecting the radial homogeneity ofthe material. To obtain large cylindrical objects, theseauthors prepared a few annular monoliths with variouswell-defined outer and inner diameters that inserted oneinto another to form a monolith with the desired largevolume. These radial flow columns extend the monolithictechnology to the field of scaled-up preparative separa-tions .
Buchmeiser et al. presentedanatypical approach tomono-lithic columns . They used ring-opening meta-thesis copolymerization of norborn-2-ene and 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalenewithin borosilicate glass columns in the presence of poro-genic solvents such as toluene, methylene chloride